各位大大，詢問一下，有人遇過CORS錯誤嗎?
我目前正在練習用SignalR，當網頁嘗試建立連線時，跑出這個錯誤，
按照微軟提供的教學，設置了一遍，但錯誤一樣存在...
[https://docs.microsoft.com/zh-tw/aspnet/core/security/cors?view=aspnetcore-3.1](在 ASP.NET Core 中啟用跨原始來源要求 (CORS))

 readonly string MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

public void ConfigureServices(IServiceCollection services)

services.AddCors(options =>
        {
            options.AddPolicy(name: MyAllowSpecificOrigins,
                              builder =>
                              {
                                  builder.WithOrigins("http://127.0.0.1:5000");
                              });
        });

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)

app.UseCors(MyAllowSpecificOrigins);

我分別新增了這兩部分
錯誤

Access to fetch at 'http://127.0.0.1:5000/MyHub/negotiate/negotiate?negotiateVersion=1' from origin 'http://localhost:5000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.